Dockerfile: update to Golang 1.12.17
Dockerfile: update to Golang 1.12.17

Signed-off-by: Sebastiaan van Stijn <64b2b6d12bfe4baae7dad3d018f8cbf6b0e7a044@gone.nl>

docker/libnetwork,mahak/libnetwork,mahak/libnetwork,docker/libnetwork,mahak/libnetwork,docker/libnetwork